St Mary's Bay, on the south coast of Newfoundland's Avalon Peninsula between Placentia Bay and Trepassey Bay, runs 65 km northeast to Colinet Harbour from its mouth between St Shotts and Point Lance, 32 km northwest. The bay branches into long harbours and inlets towards its head, with deep indentations cut into its eastern shore. Great Colinet Island, 8 km long, is situated at the centre of the bay.

Though the climate is better at the head, the two principal settlements, Branch and St Vincent's, are towards the bay's mouth, nearer the Atlantic fishing grounds. The bay was a traditional French fishing ground until the Treaty of Utrecht 1713, after which the coasts were gradually settled by English and Irish. With good forests in the north, the bay saw logging operations in addition to the fishery and some fur trapping. Today, the primary occupation of the sparsely populated bay is still fishing.